Air: May to August about 15-30°C, September to April: 0-15°C. Water: May to August fine (a shorty will do), from September to April you need a wetsuit, in December/January it can get cold (around 0°C). January to August unpredictable, can be anything. September to December usually cloudy/rainy weather though in this period, especially when there's wind.
Drive from Harderwijk across the dam over the Veluwemeer to Flevoland. Just after the dam turn right onto N306 and then onto Harderdijk (about 2.4 km after the dam).